Project Summary/Abstract We seek to improve the prediction of the risk of pathologic vertebral fracture (PVF) in patients who are treated with radiotherapy for bone metastases in the spine. Vertebral bone is the most frequent site of skeletal metastasis. Radiation therapy aims to palliate pain and reduce the risk of pathologic vertebral fracture. However, vertebral fractures are a common complication afflicting up to 39% of patients within 6 months after- radiotherapy. Clinical guidelines for estimating fracture risk remain subjective and suffer from low specificity and sensitivity. Improved assessment of fracture risk would facilitate selection of whether, how, and when to intervene prior to the occurrence of fractures. Such individualized prediction is not available in clinical practice. As part of our previous NIH grant, we have developed a computed tomography (CT) based structural analysis (CT-SAP) to successfully predict the failure of human spines with lytic defects. Based on these accomplishments, the objectives of this observational prospective study are threefold: 1) To test the performance of CT-SAP (providing a snapshot of bone structure and calcium content) and bone turnover markers (providing an indication of disease trajectory) for predicting the baseline risk of vertebral fractures in a cohort of patients treated with radiotherapy for spinal bone metastases. For this purpose, we will acquire the standard clinical CT and serum sample at the patient's radiotherapy planning. From the CT, we will derive individualized estimates of vertebral strength (CT-SAP) and vertebral loading to compute the loading / strength ratio for both treated and untreated vertebrae, and measure the value for markers for bone resorption and formation from the serum sample. We will test the independent association of the vertebral loading / strength ratio and marker value with the observed vertebral fractures within 6 months after treatment. This novel data will provide information on the effect of radiation and metastatic disease on the baseline and short term risk of vertebral fracture in this patient cohort. 2) To establish the performance of the spinal instability neoplastic score (SINS) for predicting the patient's risk of vertebral fracture within 6 months after treatment and test whether adding the load / strength ratio (CT-SAP) and bone turnover risk models, independently and combined, improves the model's performance. This will establish a new paradigm for individualized prediction of baseline risk for fracture in this patient cohort. 3) To test the established model performance for predicting the evolving risk of PVF within a 3 month interval by acquiring and analyzing the CT scans and serum samples collected at 3, 6, and 9 months after treatment as part of standard clinical care. This time period provides clinically meaningful guidelines for assessing the impact of a low vs. high risk of PVF to the health and quality of life of this infirm population with short life expectancy. Successful completion of this project will address a critical gap in our ability to individualize evaluation and management of these patients.